This invention relates generally to an antenna and, more particularly, to an antenna for transmitting and receiving electromagnetic radiation signals to and from fixed or mobile communication platforms.
The signal fading problems associated with fixed and mobile communications platforms in a multipath environment have been and continue to be studied to determine antenna and data processing designs that solve the problems in a cost-effective manner. From an antenna standpoint, previous designs have included the use of adaptive arrays and space diversity antennas. In recent years, studies have shown that frequency diversity techniques that utilize antennas with orthogonal polarization ports result in performance at least comparable to systems using space diversity.
The angular coverage desired from communications antennas, other than fixed point-to-point systems, is very large, typically equal to or approaching instantaneous hemispherical coverage. Earlier antenna designs that best achieved hemispherical coverage utilize a turnstile antenna plus a monopole antenna switched to achieve high or low angle coverage. The height of these designs range from about 0.4 to 0.5 wavelengths at the system""s operating frequency. The Rodal design is a modified version of the turnstile antenna that uses curved dipole elements and provides nearly hemispherical coverage without switching. See Rodal et al., U.S. Pat. No. 5,173,715. Rodal et al. is incorporated herein by this reference. The Rodal design is still too large for many applications with heights greater than or equal to one quarter wavelength and is a single port single polarization design.
Recently Altshuler described a simpler, non-switching design that provides hemispherical coverage. This however is not a low profile design and does not provide dual polarization outputs. See Edward E. Altshuler. Derek S. Linden, xe2x80x9cDesign of a Vehicular Antenna for GPS/Iridium Using a Genetic Algorithm.xe2x80x9d
Diversity antenna designs using crossed loop conductors have been used to combat multipath interference. See Lee, U.S. Pat. No. 4,611,212, and Johnston, et al., U.S. Pat. No. 5,784,032. Lee and Johnston et al. are incorporated herein by this reference. Both designs are narrow band designs. The Lee design is a receive antenna design. The Johnston design requires impedance matching with reactive components and does not offer the possibility of combining antenna signals to generate instantaneous hemispherical coverage.
What is needed is a low profile transmit and receive antenna design that provides (a) circular polarized hemispherical coverage using a single port output, and/or (b) orthogonal linear or circular polarized coverage using a two port output. A simple antenna design that can have an operating bandwidth  greater than 25% and that provides one or both of these modes of operation would be an improvement over the present state of the art.
It is therefore an object of this invention to provide a novel, inexpensive and highly effective low-profile antenna that is useful in both heavy multipath and minimal multipath environments. It is a further object of this invention to provide an improved, low-profile circular polarized antenna that has instantaneous coverage over a hemisphere of solid angle.
It is a further object of this invention to provide an improved, low-profile circular polarized antenna that has essentially uniform gain over a hemisphere of solid angle.
It is a further object of this invention to provide an improved, low-profile antenna that can generate a scannable, directive dual linear polarized pattern with coverage down to the horizon with scannable or switchable peaks and nulls in the azimuth plane.
It is a further object of this invention to provide an improved, low-profile two port antenna that generates dual linear polarized hemispherical coverage.
It is a further object of this invention to provide an improved, low-profile antenna with typical design dimensions of between 0.05 to 0.15 wavelengths in height by less than or equal to one-half wavelength in diameter at the desired operating frequency.
This invention results from the realization that pairs of appropriately shaped bent monopole elements that are properly oriented and properly fed form a bent monopole doublet that will provide horizon to zenith to horizon coverage. When two of these element pairs are orthogonally located and fed in phase quadrature, the result is a circular polarized antenna with hemispherical coverage. Moreover, the gain over the hemisphere can be tailored to have higher gain at low or high angles or to have uniform gain over the entire hemisphere. The two orthogonal bent monopole doublets formed provide orthogonal polarized and orthogonal angular patterns that can be processed for polarization diversity or angle diversity to mitigate multipath. If the bent monopoles are designed to be self-resonant the need for frequency bandwidth limiting reactive tuning is eliminated.
This invention most basically features an antenna comprising a ground plane having a first surface; a first pair of spaced antenna elements extending from the first surface of the ground plane; and a second pair of spaced antenna elements orthogonal to the first pair of elements and extending from the first surface of the ground plane, such that the centerpoint between each pair of antenna elements is identical. The antenna elements are preferably designed to be self-resonant. This is readily achieved by selecting the appropriate element length and geometry. Where reduced size is of greater importance than bandwidth smaller, non-resonant elements may be used with reactive tuning elements added to achieve good impedance match. At least one antenna element extends from the first surface of the ground plane and bends towards the antenna centerline that extends along an axis normal to the ground plane such that a vector representative of the element has both horizontal and vertical components as viewed against the ground plane.
The bent element, in some implementations, can be described as an asymmetric top loaded monopole with the greatest amount of top loading directed towards the antenna centerline that extends along an axis normal to the ground plane.
The first pair of antenna elements comprise first and second antenna elements, having first and second feed points respectively. The second pair of elements comprises third and fourth antenna elements having third and fourth feed points respectively. The feed points supply electrical signals to and receive electrical signals from the antenna elements. The feed points for each antenna element pair are a distance of up to and including approximately one-half wavelength apart at an operating frequency of the antenna. There may be at least one splitter/combiner having at least two input terminals and at least one output terminal. The input terminals are electrically coupled to separate antenna feed points, and the splitter/combiner is used for splitting and combining electrical signals when transmitting and receiving signals respectively.
The output ports can be combined using passive or active circuitry to achieve the desired coverage and polarization diversity.